In Patent Document 1, there is a proposal for optical glass containing 23 to 40% by mass of SiO2, 11 to 21% by mass of B2O3, etc. as optical lens for precise press having medium refractive index, low dispersion and very low softening point where refractive index (nd) is 1.55 to 1.65 and Abbe number (νd) is 54 or more. However, the optical glass of this document is not always satisfactory in weather resistance, solubility, softening property at low temperature, etc.
In Patent Document 1, there is also a description that, when phosphoric acid and boric acid are selected as oxides for the formation of glass, although they are advantageous for lowering the temperature for press molding, problems in chemical durability and weather resistance are apt to happen. Thus, amount of boric acid is made high for making the temperature for press molding low, durability and weather resistance lower corresponding thereto. Therefore, there has been almost no proposal for practical optical glass containing much amount of boric acid component up to now. Patent Document 2 proposes a composition which contains more boric acid component than Patent Document 1 but its optical constants are refractive index (nd) of 1.62 to 1.85 and Abbe number (νd) of 35 to 65 and are substantially different from those which are objects of the present invention.
Patent Document 1: JP-A-03-037130
Patent Document 2: JP-A-60-221338